In the Registration section of a membership settings, you can add new fields for information you want your members to fill out. (This information will show in your exports as well) If you click the box "Make Field Mandatory" the member will be required to fill in this field before they can register.
To view this information for a member, you will go to Memberships > Members > Click buyer's name > Participant Info. You can also edit these fields if needed.
When you add new fields to an existing membership these fields will appear within the participant info of previously registered members. You can go back and add in the information for these fields for these members at any time.